So here is the idea with some modifications/suggestions
Post a sign in a locked region, event house, something like that. The sign has a formula on it like A + B – C = X, D + E – F = Z, G – H = Y. Then have coordinates on other signs like A = (coordinate pair) and at that coord there is a sign that has the number A. Once you get all the coordinates it reveals a new location which has the actual treasure. Could be at the sky limit, could be at bedrock, would make it a adventure to find =).

The math makes it a bit more tedious to work through so it would take a while, but you could also do just regular coordinates out there in the world so people gotta race to em. Tons of other variations on this concept such as making a small scale replica world and putting a big X in red wool on the spot, then reveal the map for the players on a viewing platform. Or UO style where you get a map of a section of like coastline with landmarks and you have to figure out where the pin is.